Abstract/Summary

An important aspect of foraging ecology is the extent to which different individuals or
genders within a population exploit food resources in a different manner. For diving seabirds, much
of this information relates either to short-term dietary data or indirect measures such as time budgets.
Moreover, dietary specialisation can be difficult to detect due to biases associated with conventional
sampling techniques. We used stable isotope ratios in blood and feathers to infer trophic and habitat
specialisations among 4 diving seabird taxa—the gentoo penguin Pygoscelis papua, the macaroni
penguin Eudyptes chrysolophus, the South Georgian shag Phalacrocorax (atriceps) georgianus and
the Kerguelen shag P. (atriceps) verrucosus. This allowed us to investigate foraging specialisation
and assess whether social dominance or differences in foraging preferences explained the observed
patterns. In all taxa where sexes were known we found that males foraged at a higher trophic level
(δ15N values) than females, although this was not significant in macaroni penguins. We believe that
this is linked to a dual foraging strategy among female macaroni penguins. For South Georgian
shags, we found that sex-related dietary differences persisted for long periods (inferred from stable
isotope analyses of feathers and blood). We suggest that the trophic differences are driven by differences
in physiological performance, with males tending to dive deeper than females because of their
larger size, and hence able to access higher trophic level prey items. Moreover, male and female
shags tend to forage at different times of day; therefore, social dominance by males is unlikely to be
driving the observed differences. We also recorded highly significant relationships between stable
isotope signatures in blood (representing the breeding season diet) and those in feathers (mostly representing
the previous non-breeding season diet) in both the South Georgian and Kerguelen shags.
This strongly suggests that these 2 taxa include individuals with distinct foraging specialisation (and
most probably foraging locations) that are maintained over long periods.